1. Field of the Invention
The present invention generally relates to a power steering system for vehicles. More particularly, it relates to an electric type power steering system for vehicles.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system such as that the structure thereof is complicated, recent years have proposed a variety of electric type power steering systems for vehicles.
Exemplarily, in Japanese Patent Application Lay-Open Print No. 59-70257, laid open on Apr. 20, 1984, there is disclosed an electric power steering system for vehicles.
This electric power steering system for vehicles includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected through a universal joint with the input shaft and through a gear mechanism of a rack and pinion type with a tie rod of a steered wheel, an electric motor for supplying auxiliary torque through a reduction gearing to the output shaft, a torque detection mechanism disposed on the input shaft to detect steering torque acting on the input shaft, and a driving control circuit to produce, based on a detection signal from the torque detection mechanism, a torque magnitude signal and a torque direction signal representing the magnitude and the direction of the steering torque acting on the input shaft, respectively, and to feed the electric motor with an armature current, a proportional relation in the quantity thereof to the torque magnitude signal and in accordance in the direction of conduction thereof with the torque direction signal. The torque detection mechanism consists of a strain gauge sensor.
With such an arrangement, when the steering wheel is operated, the output shaft is applied with adequate auxiliary torque from the electric motor, so that the steering operation is facilitated.
In the electric power steering system described, however, the driving of the electric motor is controlled in such a manner that the auxiliary torque of which magnitude is substantially proportional to the that of the steering torque acting on the input shaft, is applied to the output shaft, irrespective of the speed of the rotating operation of the steering wheel.
In this respect, there is a desideratum associated with a preference that the electric motor should be controlled so as to supply auxiliary torque to the output shaft in consideration of the rotating speed of the steering wheel.
The present invention has been achieved to accomplish such desideratum in a conventional electric power steering system for vehicles.